Process and electrolyte for coloring aluminum



United States Patent 3,384,561 PROCESS AND ELECTROLYTE FOR COLORHNGALUMENUM Qhristian E. Michelson, Hamden, and David C. Montgornery,Clinton, Conn, assignors to Olin Mathieson Chemical Corporation, acorporation of Virginia N0 Drawing. Filed Feb. 15, 1965, Ser. No.432,814

9 Gaines. (Cl. 204-58) ABSTRACT OF THE DISCLOSURE The present inventionrelates to anodizing aluminum. More particularly, the present inventionresides in the production of integral, colored, anodic coatings on analuminum article by making the aluminum article anode in an electrolyticcell utilizing a particular aqueous acid electrolyte.

Decorative and protective oxide coatings on aluminum have long been madeby anodizing in electrolytes consisting of aqueous solutions of sulfuricacid. Such electrolytes are usually employed at or near room temperatureand must be cooled to maintain such temperature. The coatings soproduced are usually relatively clear or colorless, although on somealloys they may be tinted or colored by some constituents and they arecharacterized by generally poor abrasion resistance.

Processes have been developed for producing anodic oxide coatings whichare more abrasion resistant by anodizing in aqueous sulfuric acidelectrolytes at extremely low temperatures, i.e., from about 0 to 30 F.These processes are subject to the inherent disadvantage of requiringexpensive refrigeration equipment to maintain the temperatures below 30F. The coatings so produced, however, in addition to being abrasionresistant are attractively colored for numeroususes of aluminum, i.e.,are colored in dark desirable colors, such as dark brown, etc.

It is, therefore, highly desirable to develop a practical process foranodically obtaining relatively dark surface colors and good corrosionresistance.

Accordingly, it is an object of the present invention to provide apractical process as aforesaid for producing an integral, colored,anodic coating on an aluminum article and an electrolytic cell forperforming said process.

It is a further object of the present invention to provide a process asaforesaid for anodizing aluminum which yields oxide coatings havingrelatively dark surface colors.

In addition, it is an object of the present invention to provide aprocess as aforesaid which yields an integral, colored aluminum articlehaving good physical properties, such as excellent corrosion resistance,abrasion resistance and light stable colors.

It is a still further object of the present invention to provide aprocess as aforesaid which enables the attainment of a wide variety ofdesirable, integral, dark surface colors.

Further objects and advantages of the present invention will appearhereinafter.

In accordance with the process of the present invention it has now beenfound that the foregoing objects and advantages may be readily obtained.The present invention provides a process for coloring aluminumelectrolytically,

which comprises anodically oxidizing said aluminum at a current densityof from 10 to 100 amps per square foot and a voltage of from 10 to voltsin an aqueous solution consisting essentially of from 0.1 to 1% sulfuricacid, from 3% to saturation of sulfamic acid. preferably from 5 to 15%sulfamic acid, and from 0.1 to 10% sulfophthalic acid, preferably from 1to 3% sulfophthalic acid, said lath being maintained at a temperature offrom 10 to 90 It has been found in accordance with the present inventionthat the foregoing process surprisingly achieves all of the objects ofthe present invention and readily achieves a wide variety of lightstable, abrasion and corrosion resistant surface colors having excellentphysical characteristics and varying from light silver gray to jet blackor a variety of browns and dark olive shades.

It is particularly surprising that the present process attains a wideran e of highly desirable dark colors. One particularly unexpectedfeature of the present invention is the production of integral darkcolors of a relatively wide range of sulfamic acid and sulfophthalicacid concentrations but over a very narrow range of sulfuric acidconcentrations. I An additional and significant feature of the presentinvention is that all of the foregoing advantages are obtained at a lowcost which renders the process particularly attractive commercially.

It has been found that the process variables of the pirfesent inventionare important in attaining the desired e ect.

The sulfamic acid concentration may be varied over a wide range of from3% to saturation and preferably should be maintained in the range of 5%to 15%. The sulfuric acid concentration is particularly critical andshould be maintained in the range of from 0.1 to 1% by weight andpreferably from 0.25 to 0.75% by weight. The sulfophthalic acidconcentration can be varied from 0.1 to 10% and preferably is maintainedfrom 1 to 3%.

The aqueous bath is maintained at a temperature of from 10 to 90 C. andpreferably from 15 to 30 C.

The time of treatment is not especially critical, with the longer timesproviding the thicker coatings and darker colors. Generally, at leastone minute is used and preferably from 1 to 150 minutes.

The aluminum article is anodically oxidized at a current density of from10 to amps per square foot and preferably at a current density of 15 to30 amps per square foot and at a voltage of from 10 to 90 volts, andpreferably at a voltage of from 25 to 70 volts.

Naturally, various additional ingredients conventionally used may beadded to the anodic oxidation bath to achieve particular results orenhance particular characteristics, such as, for example, metalsulfates.

Any sulfophthalic acid may be employed, e.g., 4-sulfophthalic acid,S-sulfoisophthalic acid, 4-sulfophthalic acid is preferred.

After the anodic oxidation treatment, the anodized article may ifdesired be subjected to a conventional hot water sealing step, e.g.,immersion in water maintained at or near its boiling point.

The present invention is applicable to anodizing any aluminum article,i.e., high purity aluminum in various commercial grades, and aluminumbase alloys.

In .the electrolytic cell, the cathode which may be used is notespecially critical. conventionally, lead or preferably stainless steelcathodes may be used.

The present invention and the improvements resulting therefrom will bemore readily apparent from a consideration of the following illustrativeexamples wherein the amounts of all ingredients are given in percent byweight.

Example I The following example illustrates the wide variety of colorswhich may be obtained on a number of alloys in accordance with thepresent invention.

In the following example various aluminum alloys were anodicallyoxidized for varying periods of time at specified current densities andvoltages. The electrolyte used was an aqueous solution consistingessentially of about 0.25% sulfuric acid, about 8% sulfamic acid andabout 2% 4-sulfophthalic acid. The bath was maintained at a temperatureof 25 C. The cathode used was lead. The voltage was raised to maintainthe cur-rent density until the maximum voltage was reached. The voltagewas then held constant until the anodizing time was completed. Airagitation of the electrolyte was used.

The results are shown in Table I below:

The following example illustrates the abrasion resistance of the samplesanodized in accordance with the process of the present invention.

Samples anodized in a manner after Example I were tested for abrasionresistance by abrading the surface of the sample with a fine jet ofsand. The time necessary to penetrate the anodic coating in ten spots oneach specimen was measured. The total time to penetrate the ten 10)spots was then divided by the coating thickness in mils, giving a figureof merit the units of which are seconds per mil. The higher the secondsper mil, the greater the abrasion resistance.

The data is shown in the following table.

TABLE II Abrasion Alloy Processing Resistance,

Secs/Mil.

30-rnin. auodize 165 g./l. H2SO4-12 ASF 25 1100 30-min. anodize 165g./l. 11280 -24 ASF. 32 1100- 30-min. anodize 8% sulfamic acid plus0.25%

IgSFO4 plus 2% at-suliophathalic acid-24 Example III The resultingcolors obtained are shown in photovolt readings wherein generally darkercolors are indicated by low photovolt readings, i.e., the lowerphotovolt reading indicates lower reflectivity, hence a darker color.

The results, shown in Table III below, show graphically the darkercolors obtained in accordance with the process of the present invention.

This invention may be embodied in other forms or carried out in otherways without departing from the spirit or essential characteristicsthereof. The present embodiment is therefore to be considered as in allrespects illustrative and not restrictive, the scope of the inventionbeing indicated by the appended claims, and all changes which comewithin the meaning and range of equivalency are intended to be embracedtherein.

What is claimed is:

1. A process for coloring aluminum electrolytically which comprisesanodically oxidizing said aluminum at a current density of from 10 toamps per square foot and a voltage of from 10 to 90 volts in an aqueoussolution consisting essentially of from 0.1 to 1% sulfuric acid, from 3%to saturation of sulfamic acid, and from 0.1 to 10% sulfophthalic acid,said bath being maintained at a temperature of from 10 to 90 C.

2. A process according to claim 1 wherein the sulfamic acid is presentin a concentration of from 5 to 15%.

3. A process according to claim 1 wherein the sulfuric acid is presentin a concentration of from 0.25 to 0.75%.

4. A process according to claim 1 wherein the sulfophtha-lic acid ispresent in a concentration of from 1 to 3%.

5. A process according to claim 11 wherein said bath is maintained at atemperature of from 15 to 30 C.

6. A process according to claim 1 wherein the current density is from 15to 30 amps per square foot and the voltage is from 25 to 70 volts.

7. A process according to claim 1 wherein the sulfophth'alic acid is4-sulfophthalic acid.

8. A process according to claim 1 wherein said aluminum is anodicallyoxidized for from 1 to minutes.

9. An electrolyte for coloring aluminum electrolytically comprising anaqueous solution consisting essentially of from 0.1 to 1% sulfuric acid,from 3% saturation of sulfaniic acid and from 0.1 to 10% sulfophthalicacid.

References Cited UNITED STATES PATENTS 2,855,352 10/1958 Ernst 204583,227,639 1/ l966 Kampert 204-58 JOHN H. MACK, Primary Examiner.

HOWARD S. WILLIAMS, Examiner. G. KAPLAN, Assistant Examiner.

